Vendio is my favorite auction management tool. They provide inventory control, scheduling, photo hosting, templates, auto feedback posting and more.
I use (and sell) the EZ Cube Digital Photo Light Tents to take great photos for my eBay listings.
The Firefox toolbar for eBay is an excellent Firefox add-on that lets you quickly access things like your eBay auctions, price comparisons for various products and customer reviews. IE users can use the default toolbar provided by eBay which has similar features.
Wipbox is a very useful tool for eBay sellers. It makes posting to eBay easier and also helps you in researching things like average prices of products and customer reviews. It also lets you upload photos related to your eBay auctions and store them for free.
Auction Sniper is my favorite eBay sniping tool
RSSAuction is a neat tool that lets you create RSS feeds for custom searches in eBay. Hence it makes tracking a particular item on eBay a breeze.
PicClick is an awesome visual search engine for eBay which that lets you browse product images based on categories, price range and listing or ending time of bidding.